In fluid process control applications in chemical, pulp, food, and other fluid processing plants, different types of pressure transmitters are used. These types generally include absolute pressure transmitters that measure a process pressure relative to a vacuum; gauge pressure transmitters that measure a process pressure relative to local atmospheric pressure; and differential pressure transmitters that measure a difference between two process pressures. Pressure transmitters also typically measure pressure over a limited range with a specified accuracy. Typically a pressure transmitter will be manufactured in two or more overlapping ranges, each specified to measure pressure accurately over about a 100:1 turndown range to fill application needs up to approximately ten thousand pounds per square inch.
Differential pressure transmitters, in particular, are designed for specific pressure ranges and have limits as to how far the differential pressure transmitter can be ranged down. Further, many applications also require knowledge of the line pressure of the monitored process. For example, commercially available devices, such as the Model 3095 MV available from Rosemount Inc., of Chanhassen, Minn., measures differential pressure and line pressure in order to execute a flow calculation to provide process fluid flow measurement. While the use of a single absolute or gauge pressure sensor in conjunction with a differential pressure sensor has provided advantages in the past, such devices can cease to function if the absolute pressure sensor, or the differential pressure sensor fails, or if any of the pressures coupled to the pressure transmitter are outside of the selected measurement ranges.